Lidar is considered to be a key sensing technology in all walks of life, and it acts as a promoter in fields such as robotics, unmanned driving, and smart cities. In recent years, the solid-state lidar, which has been high hopes, has become a hot spot in the industry.
What is solid-state lidar?
In theory, solid-state lidar is a radar with no moving parts at all. Optical Phased Array and Flash are its typical technical routes, and are also considered pure solid-state lidar solutions.
However, in recent years, some non-fully rotating lidars have also been collectively referred to as "solid-state lidars". They have many performance characteristics of solid-state lidars, such as high resolution and limited horizontal FOV (forward instead of 360°). However, these technical solutions have some tiny moving parts, which cannot be regarded as pure solid-state lidar in a strict sense.
How solid-state lidar works
Solid-state lidar mainly relies on the reflection or reception of waves to detect the characteristics of the target. Most of it comes from the research of three-dimensional image sensors. It actually comes from the infrared focal plane imager. The focal plane of the focal plane detector is arranged with an array of photosensitive elements. Infrared rays emitted at infinity are imaged on these photosensitive elements in the focal plane of the system through the optical system. The detector converts the received optical signals into electrical signals, performs integral amplification, sample and hold, and finally sends them through the output buffer and multiplex transmission system. Up to the surveillance system to form an image.
Three technical routes formed by solid-state lidar
After years of development, the basic framework of solid-state lidar has been relatively clear. The following are the current mainstream three solutions.
1. MEMS (Micro-Electro-Mechanical System) Micro-Electro-Mechanical System
MEMS refers to the miniaturization and electronic design of mechanical mechanisms, and the integration of large mechanical structures on silicon-based chips through microelectronic processes for mass production. The technology is mature and can be mass produced. The vertical one-dimensional scanning is mainly achieved through MEMS micromirrors. The whole machine rotates 360 degrees to complete the horizontal scanning. The light source is a fiber laser. This is mainly due to the low repetition frequency of the 905-nanometer tube. A high average power will be too large, which will affect the life of the laser tube.
Strictly speaking, MEMS is not a pure solid-state lidar. This is because the MEMS solution does not completely eliminate the machinery, but the machinery is miniaturized, and the scanning unit becomes a MEMS micromirror.
2. OPA (optical phased array) optical phased array technology
Compared with other technical solutions, the OPA solution describes the bright prospect of a lidar chip-level solution. It mainly uses multiple light sources to form an array, and controls the light-emitting time difference of each light source to synthesize the main beam with a specific direction. Then control it, and the main beam can scan in different directions. The accuracy of the radar can reach the millimeter level, and it conforms to the trend of solid-state, miniaturization and low-cost lidar in the future, but the difficulty lies in how to improve the point cloud data measured per unit time and the huge investment cost.
3.Flash
The principle of Flash Lidar is also fast flash. It is not like MEMS or OPA which will scan, but directly emits a large area of laser covering the detection area in a short time, and then uses a highly sensitive receiver to complete the surrounding environment The drawing of the image.
The advantages and disadvantages of solid-state lidar
The solid-state lidar using optical phased array scanning technology does have many advantages, such as:
① Its structure is simple, the size is small, no rotating parts are needed, and the structure and size can be greatly compressed, which improves the service life and reduces its cost.
②The scanning accuracy is high. The scanning accuracy of the optical phased array depends on the accuracy of the control electrical signal, which can reach the order of one-thousandth.
③It has good controllability, can be pointed arbitrarily within the allowable angle range, and can perform high-density scanning in key areas.
④ The scanning speed is fast. The scanning speed of the optical phased array depends on the electronic characteristics of the materials used, and can generally reach the MHz level.
Of course, solid-state lidar also has some disadvantages, such as:
① The scan angle is limited, and the solid state means that the lidar cannot rotate 360 degrees and can only detect the front. Therefore, to achieve omni-directional scanning, multiple solid-state lidars (at least two front and back) must be arranged in different directions
②Sidelobe problem, grating diffraction will also form other bright patterns in addition to the central pattern. This problem will cause the laser to form sidelobes in the direction of the maximum power, dispersing the energy of the laser.
③The processing difficulty is high. The optical phased array requires that the size of the array element must not be greater than half a wavelength. Generally, the current working wavelength of lidar is about 1 micron, so the size of the array element must not be greater than 500nm. Moreover, the higher the array density, the more concentrated the energy, which raises the requirements for processing accuracy and requires certain technological breakthroughs.
④ Large receiving area and poor signal-to-noise ratio: Traditional mechanical radars only need a small receiving window, but solid-state lidars need an entire receiving surface, so more ambient light noise is introduced, which increases the difficulty of scanning analysis.
Generally speaking, at present, the existing radar products on the market are difficult to meet the existing characteristics of solid-state lidar (high reliability, low cost and long range), which also determines the solid-state lidar It is difficult to be commercialized in a short time. At the same time, the delivery dates of all solid-state radar companies are currently being extended.
Although many people in the industry predict that solid-state, miniaturization, and low cost will be the future development trend of lidar , at present, mechanical lidar is still the mainstream.